In a wavelength division multiplexed (WDM) optical link, one and the same optical path between a transmitter and a receiver is used multiple times by sending multiple data channels over it using light with different frequencies. To this end, a light source generates a plurality of discrete lines with different frequencies. The lines are modulated according to a data stream separately from one another, so that each line supports an independent channel for the transmission of data. Such links are well known in the art, for example, from (A. Akrout, A. Shen, R. Brenot, F. van Dijk, O. Legouezigou, F. Pommereau, F. Lelarge, A. Ramdane, G.-H. Duan, “Separate Error-Free Transmission of Eight Channels at 10 Gb/s Using Comb Generation in a Quantum-Dash-Based Mode-Locked Laser”, IEEE Photonics Technology Letters 21 (23), 1746-1748 (2009)).
If a separate laser is used to generate each line, the transmitter becomes bulky, expensive and consumes a large amount of electrical power. If a single source is used to generate all frequencies simultaneously, however, all generated lines share the limited total power of the source. Since much of the power is lost during modulation or due to interfaces between optical elements, the link budget is limited. To improve the link (e.g., to increase the range or to improve the error rate), optical amplifiers that simultaneously amplify all lines of the light are used. Examples for such amplifiers are given in (D. Yin, A. Gubenko, I. Krestnikov, D. Livshits, S. Mikhrin, A. Kovsh, G. Wojcik, “Laser Diode Comb Spectrum Amplification Preserving Low RIN for WDM applications”, SPIE-OSA-IEEE 7631, 76311R-1 to 76311R-7 (2009)) as semiconductor optical amplifiers (SOAs).
The main quantities of interest regarding the quality of the link are the achievable range and the error rate. Using a SOA as an optical amplifier necessitates a trade-off regarding the power per channel. While a higher power per channel increases the achievable range and increases the signal-to-noise ratio (also in view of the excess noise generated by the SOA), which in turn improves the error rate, the signal is distorted if the amplifier is operated beyond the linear regime. If the amplifier is operated in its linear regime, however, the power per channel at the output of the SOA is limited. Moreover, there is a tradeoff between the number of jointly amplified channels and the achievable power per channel at the output of the SOA while maintaining the SOA in its linear regime, since the total output power of the SOA is limited. While amplification of an optical channel can be performed with a saturated erbium-doped fiber amplifier (EDFA) without distorting the data, EDFAs are also bulky, expensive and power hungry compared to SOAs. In the case of EDFAs or comparable erbium doped waveguide amplifiers, suboptimum amplification also leads, e.g., to increased power consumption.
Modulation of individual channels in a WDM system can be obtained by means of WDM multiplexers or by means of frequency selective modulators. A well-known problem with frequency selective modulators is, however, that their frequency of operation is generally different from the frequency of lines targeted for modulation unless they are being actively tuned. Such tuning results in additional power consumption. Methods to facilitate tuning of frequency selective modulators are well known in the art. One such method is for example described in (Y. Zheng, P. Lisherness, M. Gao, J. Bovington, S. Yang, K.-T. Cheng, “Power-Efficient Calibration and Reconfiguration for On-Chip Optical Communication,” Proc. of the Design, Automation & Test in Europe Conference, 1501 to 1506, (2012)). A drawback of this method is that the free spectral range of the frequency selective modulators needs to be close to the difference between the frequencies of the highest and lowest frequency channels.